diff --git a/codex-rs/Cargo.lock b/codex-rs/Cargo.lock
index dfae092ab7..2e2ec60434 100644
--- a/codex-rs/Cargo.lock
+++ b/codex-rs/Cargo.lock
@@ -5400,6 +5400,19 @@ dependencies = [
  "toml 0.9.11+spec-1.1.0",
 ]
 
+[[package]]
+name = "codex-utils-model-facing-id"
+version = "0.0.0"
+dependencies = [
+ "blake3",
+ "pretty_assertions",
+ "schemars 0.8.22",
+ "serde",
+ "serde_json",
+ "thiserror 2.0.18",
+ "tracing",
+]
+
 [[package]]
 name = "codex-utils-oss"
 version = "0.0.0"
diff --git a/codex-rs/Cargo.toml b/codex-rs/Cargo.toml
index cbd41d41d2..0acadbfe5c 100644
--- a/codex-rs/Cargo.toml
+++ b/codex-rs/Cargo.toml
@@ -147,6 +147,7 @@ members = [
     "utils/cli",
     "utils/elapsed",
     "utils/sandbox-summary",
+    "utils/model-facing-id",
     "utils/sleep-inhibitor",
     "utils/approval-presets",
     "utils/oss",
diff --git a/codex-rs/utils/model-facing-id/BUILD.bazel b/codex-rs/utils/model-facing-id/BUILD.bazel
new file mode 100644
index 0000000000..8e195c83f6
--- /dev/null
+++ b/codex-rs/utils/model-facing-id/BUILD.bazel
@@ -0,0 +1,6 @@
+load("//:defs.bzl", "codex_rust_crate")
+
+codex_rust_crate(
+    name = "model-facing-id",
+    crate_name = "codex_utils_model_facing_id",
+)
diff --git a/codex-rs/utils/model-facing-id/Cargo.toml b/codex-rs/utils/model-facing-id/Cargo.toml
new file mode 100644
index 0000000000..587da05235
--- /dev/null
+++ b/codex-rs/utils/model-facing-id/Cargo.toml
@@ -0,0 +1,23 @@
+[package]
+name = "codex-utils-model-facing-id"
+version.workspace = true
+edition.workspace = true
+license.workspace = true
+
+[lints]
+workspace = true
+
+[lib]
+doctest = false
+test = false
+
+[dependencies]
+blake3.workspace = true
+schemars.workspace = true
+serde = { workspace = true, features = ["derive"] }
+thiserror.workspace = true
+tracing.workspace = true
+
+[dev-dependencies]
+serde_json.workspace = true
+pretty_assertions.workspace = true
diff --git a/codex-rs/utils/model-facing-id/src/lib.rs b/codex-rs/utils/model-facing-id/src/lib.rs
new file mode 100644
index 0000000000..c93dbd2f48
--- /dev/null
+++ b/codex-rs/utils/model-facing-id/src/lib.rs
@@ -0,0 +1,197 @@
+//! Issues a salt followed by shuffled decimal IDs from `000`–`999`, then `001000`–`999999`.
+//! Each pool uses a four-round Feistel permutation with BLAKE3 and cycle-walking.
+//! The seed can be varied across sessions to change the order in which IDs are issued.
+
+use std::borrow::Cow;
+use std::fmt;
+use std::ops::RangeInclusive;
+
+use schemars::JsonSchema;
+use serde::Deserialize;
+use serde::Serialize;
+use thiserror::Error;
+
+const VERSION: u32 = 1;
+// Our tokenizers encode every decimal ID of up to three digits as a single token.
+const GROUP_SIZE: u64 = 1_000;
+
+/// Stores the salt and numeric value separately. Display pads the value to groups of three digits.
+/// With salt `environment`, zero renders as `environment000` and 1000 as `environment001000`.
+#[derive(Clone, Debug, PartialEq, Eq, PartialOrd, Ord, Hash, Serialize, Deserialize)]
+pub struct ModelFacingId {
+    salt: Cow<'static, str>,
+    value: u64,
+}
+
+impl fmt::Display for ModelFacingId {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "{}{}", self.salt, self.digits())
+    }
+}
+
+impl ModelFacingId {
+    /// Formats the padded digits for protocols that supply their own type prefix.
+    pub fn digits(&self) -> impl fmt::Display + '_ {
+        fmt::from_fn(|f| {
+            write!(
+                f,
+                "{:0width$}",
+                self.value,
+                width = decimal_width(self.value)
+            )
+        })
+    }
+}
+
+fn decimal_width(value: u64) -> usize {
+    (value.checked_ilog10().unwrap_or(0) as usize + 1).next_multiple_of(3)
+}
+
+/// Generates token-efficient identifiers for model context.
+#[derive(Debug)]
+pub struct ModelFacingIdGenerator {
+    state: ModelFacingIdGeneratorState,
+}
+
+impl ModelFacingIdGenerator {
+    /// Starts a sequence for one ID kind using a shared session seed.
+    /// The salt also prefixes each displayed ID. Use the [tokenizer](https://platform.openai.com/tokenizer)
+    /// to choose a stable, single-token salt such as `environment`.
+    ///
+    /// # Panics
+    ///
+    /// Panics if the salt is empty.
+    pub fn new(seed: u64, salt: &'static str) -> Self {
+        assert!(!salt.is_empty(), "a model ID salt must be nonempty");
+        let mut hasher = blake3::Hasher::new();
+        hasher.update(&seed.to_le_bytes());
+        hasher.update(salt.as_bytes());
+        let mut seed_bytes = [0; 8];
+        hasher.finalize_xof().fill(&mut seed_bytes);
+        let derived_seed = u64::from_le_bytes(seed_bytes);
+        Self {
+            state: ModelFacingIdGeneratorState::new(
+                VERSION,
+                derived_seed,
+                salt,
+                /*next_ordinal*/ 0,
+            ),
+        }
+    }
+
+    /// Issues the next ID. After all `u64` values are issued, the sequence repeats from zero.
+    pub fn fetch_next(&mut self) -> ModelFacingId {
+        let ordinal = self.state.next_ordinal;
+        let mut pool = 0..=GROUP_SIZE - 1;
+        while ordinal > *pool.end() {
+            let start = pool.end() + 1;
+            // The final decimal pool ends at u64::MAX, which has no representable successor.
+            let last = start
+                .checked_mul(GROUP_SIZE)
+                .map_or(u64::MAX, |end| end - 1);
+            pool = start..=last;
+        }
+        let id = permute(self.state.seed, pool, ordinal);
+        let (next, wrapped) = ordinal.overflowing_add(1);
+        if wrapped {
+            tracing::error!(
+                "model-facing ID generator exhausted all u64 values and wrapped to zero"
+            );
+        }
+        self.state.next_ordinal = next;
+        ModelFacingId {
+            salt: self.state.salt.clone(),
+            value: id,
+        }
+    }
+
+    /// Captures the position of the next allocation.
+    pub fn snapshot(&self) -> ModelFacingIdGeneratorState {
+        self.state.clone()
+    }
+
+    /// Continues the snapshot's sequence from its next unissued ordinal.
+    pub fn restore(state: ModelFacingIdGeneratorState) -> Result<Self, RestoreError> {
+        if state.version != VERSION {
+            return Err(RestoreError::UnsupportedVersion(state.version));
+        }
+        if state.salt.is_empty() {
+            return Err(RestoreError::InvalidSalt);
+        }
+        Ok(Self { state })
+    }
+}
+
+/// Persisted sequence version, derived seed, salt, and zero-based next allocation ordinal.
+#[derive(Clone, Debug, PartialEq, Eq, Serialize, Deserialize, JsonSchema)]
+pub struct ModelFacingIdGeneratorState {
+    version: u32,
+    seed: u64,
+    salt: Cow<'static, str>,
+    next_ordinal: u64,
+}
+
+impl ModelFacingIdGeneratorState {
+    /// Returns the namespace this snapshot belongs to.
+    pub fn salt(&self) -> &str {
+        &self.salt
+    }
+
+    /// Rebuilds a snapshot from its stored fields. [`ModelFacingIdGenerator::restore`] validates it.
+    pub fn new(
+        version: u32,
+        seed: u64,
+        salt: impl Into<Cow<'static, str>>,
+        next_ordinal: u64,
+    ) -> Self {
+        Self {
+            version,
+            seed,
+            salt: salt.into(),
+            next_ordinal,
+        }
+    }
+}
+
+/// The snapshot cannot be used to resume an allocation sequence.
+#[derive(Debug, PartialEq, Eq, Error)]
+pub enum RestoreError {
+    #[error("unsupported model-facing ID generator version {0}")]
+    UnsupportedVersion(u32),
+    #[error("a model ID salt must be nonempty")]
+    InvalidSalt,
+}
+
+// Use the smallest balanced Feistel domain that covers the pool to limit cycle-walking.
+// Both the four rounds and their input encoding are part of the snapshot format.
+fn permute(seed: u64, pool: RangeInclusive<u64>, ordinal: u64) -> u64 {
+    let start = *pool.start();
+    let size = pool.end() - start + 1;
+    let bits = u64::BITS - (size - 1).leading_zeros();
+    let half_bits = bits.div_ceil(2);
+    let half_mask = (1_u64 << half_bits) - 1;
+    // The seed, pool index, and round occupy 17 bytes. The pool width fixes the right half's length.
+    let input_len = 17 + half_bits.div_ceil(8) as usize;
+    let mut input = [0; 25];
+    input[..8].copy_from_slice(&seed.to_le_bytes());
+    input[8..16].copy_from_slice(&(start / GROUP_SIZE).to_le_bytes());
+
+    let mut slot = ordinal - start;
+    loop {
+        let (mut left, mut right) = (slot >> half_bits, slot & half_mask);
+        for round in 0..4 {
+            input[16] = round;
+            input[17..].copy_from_slice(&right.to_le_bytes());
+            let hash = blake3::hash(&input[..input_len]);
+            let mut bytes = [0; 8];
+            bytes.copy_from_slice(&hash.as_bytes()[..8]);
+            let f = u64::from_le_bytes(bytes) & half_mask;
+            (left, right) = (right, left ^ f);
+        }
+        slot = (left << half_bits) | right;
+        // Following the permutation cycle must eventually return to the starting slot.
+        if slot < size {
+            return start + slot;
+        }
+    }
+}
diff --git a/codex-rs/utils/model-facing-id/tests/generator.rs b/codex-rs/utils/model-facing-id/tests/generator.rs
new file mode 100644
index 0000000000..805c6e9461
--- /dev/null
+++ b/codex-rs/utils/model-facing-id/tests/generator.rs
@@ -0,0 +1,193 @@
+//! Checks pool boundaries, snapshot restore, and the v1 sequence through the public API.
+
+use std::collections::BTreeSet;
+use std::error::Error;
+
+use codex_utils_model_facing_id::ModelFacingId;
+use codex_utils_model_facing_id::ModelFacingIdGenerator;
+use codex_utils_model_facing_id::ModelFacingIdGeneratorState;
+use codex_utils_model_facing_id::RestoreError;
+use pretty_assertions::assert_eq;
+use serde_json::json;
+
+/// Every three-digit suffix is issued before the first ID drawn from the six-digit pool.
+#[test]
+fn exhausts_three_digit_ids_before_opening_the_six_digit_pool() {
+    let mut generator = ModelFacingIdGenerator::new(/*seed*/ 42, "environment");
+    let mut actual = BTreeSet::new();
+    for _ in 0..1_000 {
+        actual.insert(generator.fetch_next().to_string());
+    }
+    let expected: BTreeSet<_> = (0..1_000).map(|id| format!("environment{id:03}")).collect();
+    assert_eq!(actual, expected);
+    assert_eq!(generator.fetch_next().to_string(), "environment825109");
+}
+
+/// IDs can have the same display text and still round-trip with distinct salts and values.
+#[test]
+fn persisted_ids_keep_the_salt_and_value_separate() -> Result<(), serde_json::Error> {
+    for (salt, value) in [("env123", 0), ("env", 123_000)] {
+        let saved = json!({ "salt": salt, "value": value });
+        let id: ModelFacingId = serde_json::from_value(saved.clone())?;
+        assert_eq!(id.to_string(), "env123000");
+        assert_eq!(serde_json::to_value(id)?, saved);
+    }
+    Ok(())
+}
+
+/// A saved cursor resumes the uninterrupted sequence both inside and between pools.
+#[test]
+fn serialized_snapshots_resume_the_sequence() -> Result<(), Box<dyn Error>> {
+    let derived_seed = 0x9e6e_7c74_2945_897e;
+    for next_ordinal in [437, 1_000, 1_437] {
+        let mut original = ModelFacingIdGenerator::new(u64::MAX, "env2");
+        for _ in 0..next_ordinal {
+            original.fetch_next();
+        }
+        let snapshot = original.snapshot();
+        assert_eq!(
+            ModelFacingIdGeneratorState::new(
+                /*version*/ 1,
+                derived_seed,
+                "env2",
+                next_ordinal
+            ),
+            snapshot
+        );
+        let saved = serde_json::to_value(snapshot)?;
+        assert_eq!(
+            saved,
+            json!({"version": 1, "seed": derived_seed, "salt": "env2", "next_ordinal": next_ordinal})
+        );
+        let mut resumed = ModelFacingIdGenerator::restore(serde_json::from_value(saved)?)?;
+        for _ in 0..1_001 {
+            assert_eq!(resumed.fetch_next(), original.fetch_next());
+        }
+    }
+    Ok(())
+}
+
+/// V1 must keep the initial outputs and repeated cycle walks stable.
+#[test]
+fn v1_known_outputs() -> Result<(), Box<dyn Error>> {
+    let seed = 0x1234_5678_9abc_def0;
+    let state = ModelFacingIdGeneratorState::new(
+        /*version*/ 1,
+        seed,
+        "environment",
+        /*next_ordinal*/ 0,
+    );
+    let mut generator = ModelFacingIdGenerator::restore(state)?;
+    for id in ["895", "930", "155", "141", "916", "912", "678", "430"] {
+        assert_eq!(
+            generator.fetch_next().to_string(),
+            format!("environment{id}")
+        );
+    }
+    // Ordinal 1,605 visits two excluded slots before returning to the six-digit pool.
+    let state = ModelFacingIdGeneratorState::new(
+        /*version*/ 1,
+        seed,
+        "environment",
+        /*next_ordinal*/ 1_605,
+    );
+    let mut resumed = ModelFacingIdGenerator::restore(state)?;
+    assert_eq!(resumed.fetch_next().to_string(), "environment765577");
+    Ok(())
+}
+
+/// The last ID at each width and the first at the next width stay stable across restores.
+#[test]
+fn v1_known_outputs_at_every_pool_boundary() -> Result<(), Box<dyn Error>> {
+    let seed = 0x1234_5678_9abc_def0;
+    for (boundary, last, first) in [
+        (1_000, "553", "661326"),
+        (1_000_000, "753874", "286136338"),
+        (1_000_000_000, "839882170", "813648058018"),
+        (1_000_000_000_000, "634462456979", "134726697070558"),
+        (
+            1_000_000_000_000_000,
+            "326088534089890",
+            "563366069705075074",
+        ),
+        (
+            1_000_000_000_000_000_000,
+            "633936232025849076",
+            "012827849051678362461",
+        ),
+    ] {
+        let state = ModelFacingIdGeneratorState::new(/*version*/ 1, seed, "env", boundary - 1);
+        let mut generator = ModelFacingIdGenerator::restore(state)?;
+        assert_eq!(generator.fetch_next().to_string(), format!("env{last}"));
+        let mut resumed = ModelFacingIdGenerator::restore(generator.snapshot())?;
+        let next = generator.fetch_next();
+        assert_eq!(next.to_string(), format!("env{first}"));
+        assert_eq!(resumed.fetch_next(), next);
+    }
+    Ok(())
+}
+
+/// The same session seed can drive several ID kinds, each with its own resumable seed.
+#[test]
+fn salts_derive_the_seed_stored_in_snapshots() {
+    for (seed, salt, derived_seed) in [
+        (42, "environment", 0xc9b5_d0ac_9722_ea42),
+        (42, "context-window", 0x2747_db8a_6513_485b),
+        (43, "environment", 0x70aa_a9fd_4bf2_6e41),
+    ] {
+        assert_eq!(
+            ModelFacingIdGenerator::new(seed, salt).snapshot(),
+            ModelFacingIdGeneratorState::new(
+                /*version*/ 1,
+                derived_seed,
+                salt,
+                /*next_ordinal*/ 0
+            )
+        );
+    }
+}
+
+/// The final ordinal belongs to the last decimal pool. Its successor restarts the sequence.
+#[test]
+fn wraps_after_the_last_u64_ordinal() -> Result<(), Box<dyn Error>> {
+    let seed = 17;
+    let last = ModelFacingIdGeneratorState::new(/*version*/ 1, seed, "env", u64::MAX);
+    let mut generator = ModelFacingIdGenerator::restore(last)?;
+    let text = generator.fetch_next().to_string();
+    let last_id = text
+        .strip_prefix("env")
+        .expect("restored salt")
+        .parse::<u64>()?;
+    assert!(last_id >= 1_000_000_000_000_000_000);
+
+    let first =
+        ModelFacingIdGeneratorState::new(/*version*/ 1, seed, "env", /*next_ordinal*/ 0);
+    assert_eq!(generator.snapshot(), first);
+    let mut fresh = ModelFacingIdGenerator::restore(first)?;
+    let mut resumed = ModelFacingIdGenerator::restore(generator.snapshot())?;
+    assert_eq!(resumed.fetch_next(), fresh.fetch_next());
+    assert_eq!(resumed.snapshot(), fresh.snapshot());
+    Ok(())
+}
+
+#[test]
+fn rejects_unsupported_snapshot_versions() {
+    let saved = ModelFacingIdGeneratorState::new(
+        /*version*/ 2, /*seed*/ 0, "env", /*next_ordinal*/ 0,
+    );
+    assert_eq!(
+        ModelFacingIdGenerator::restore(saved).unwrap_err(),
+        RestoreError::UnsupportedVersion(2)
+    );
+}
+
+#[test]
+fn rejects_empty_salts_on_restore() {
+    let state = ModelFacingIdGeneratorState::new(
+        /*version*/ 1, /*seed*/ 42, "", /*next_ordinal*/ 0,
+    );
+    assert_eq!(
+        ModelFacingIdGenerator::restore(state).expect_err("empty salt"),
+        RestoreError::InvalidSalt
+    );
+}